System for detecting linear dimensions and method for testing the system operability

ABSTRACT

A measuring system for a numerical control machine tool comprising a detecting head (3), for contacting a workpiece (1) to be checked and for transmitting an optical coded signal (8) to a remote transceiver unit (11) that, in turn, can transmit an optical switch on signal (10) to the head (3). The transceiver unit (11) is connected, by means of a cable (45) including separate lines (43, 44) for the transmitted and received signals, to an interface unit (12), in turn connected to a computerized numerical control unit (13). In the event that faults occur in the transmission between the head (3) and the interface unit (12), a test takes place so as to identify in which part of the system the fault is located. A test signal is generated by suitable means (38) in the interface unit (12), transmitted from the transceiver unit (11) towards a reflecting surface (16), reflected by the latter, received by the transceiver unit (11) and sent to the interface unit (12). If the test outcome is positive, it means that head (3) is the cause for the malfunction. In the contrary, the test continues, by shortcircuiting the connection lines (43, 44) first to one, then to the other end of cable (45) for excluding from the test the transceiver unit (11), and cable (45) itself.

TECHNICAL FIELD

The present invention relates to a system for checking the dimensions ofmechanical workpieces, including a detecting apparatus and a processingand control unit, the detecting apparatus comprising: a detecting head,for cooperating with the workpiece to be checked, with transmissiondevices for transmitting first electromagnetic coded signals; atransceiver unit, separate from the head, with a receiving section forreceiving the first electromagnetic coded signals, and a transmittingsection for transmitting second electromagnetic signals; and aninterface unit, connected to the transceiver unit and to the processingand control unit, comprising reception means and means for generatingswitch on signals.

The invention also relates to a method for testing the operability of asystem for checking the dimensions of mechanical workpieces including adetecting head for transmitting first electromagnetic coded signals anda transceiver unit, separate from the head, with a receiving section forreceiving the first electromagnetic coded signals and a transmittingsection for transmitting second electromagnetic signals.

BACKGROUND ART

In known measuring systems like those applied, for example, in numericalcontrol machine tools for detecting the position and/or the dimensionsof machined workpieces a contact detecting head, mounted on the machine,in the course of a checking cycle displaces with respect to theworkpiece, touches the surface to be checked and, further to makingcontact, sends wireless signals to an appropriate receiving unit. Thereceiving unit is in turn connected, by means of an interface device, tothe numerical control unit that, by processing other signals indicativeof the position of the head, obtains the information about the positionof the workpiece surface.

The system can foresee the head to be power supplied by an electricbattery and be kept under normal circumstances in a "stand by" conditionof low electric power consumption and that, when there is the need toperform a measuring cycle, a wireless switch on signal be sent by thereceiving unit to the transmission circuits of the head to activate thepower supply, by means of appropriate switching devices sensitive to theswitch on signal. When the measuring cycle ends, the receiving unitsends a wireless switch off signal that brings the head back to the"stand by" low consumption condition.

A system of this type is illustrated and described in US patent U.S.Pat. No. 4,693,110. This patent also discloses a method and an apparatusfor testing the operability conditions of the head.

Whenever, in the course of a measuring cycle, there are some doubtsabout the system operating correctly, in particular whenevertransmission between head and interface appears to be interrupted, it isnecessary to identify the origin of the malfunction, that may affect thehead transmitter, the receiving unit, the interface device, or even theconnecting cables.

Under this circumstance all the component parts are inspected until thefault is detected; this involves the testing of the associated circuitsby accomplishing delicate operations that foresee the use of appropriateinstruments and often the attendance of specialized personnel.

DISCLOSURE OF THE INVENTION

Object of the present invention is to provide a system that, in theevent a malfunction occurs, can locate in a simple and rapid way whichis the faulty component.

This object is achieved by a system wherein the interface unit comprisesmeans for generating test signals, the transmitting section and thereceiving section of the transceiver unit being adapted for transmittingassociated electromagnetic test signals and for receiving reflectedelectromagnetic test signals, respectively, for testing the operabilityof the detecting apparatus.

A further object of the invention is the providing of a particularlysimple methodical test of the operability condition of the system.

This further object is achieved by a method comprising the steps of:commanding the transceiver unit to transmit second electromagneticsignals; arranging a reflecting surface next to the transmitting sectionso as to cause the reflection of the electromagnetic signals towards thereceiving section of the transceiver unit; and testing that thereceiving section has correctly received the reflected electromagneticsignals.

An important advantage that a system and a method according to theinvention provide is that of enabling the user to locate the fault byperforming a few simple operations, without there being the need toapply for the assistance of specialized personnel or require specificinstrumentation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is hereinafter described in more detail with reference tothe enclosed sheets of drawings, given by way of non-limiting exampleonly, wherein

FIG. 1 is an extremely schematic view of a system according to theinvention;

FIG. 2 is an enlarged scale, longitudinal sectional view, angularlydisplaced by 90°, of a device belonging to the system shown in FIG. 1,

FIG. 3 is a front view of the device of FIG. 2,

FIG. 4 is a block diagram indicating the functions of some components ofthe system shown in FIG. 1, and

FIG. 5 is a logic block diagram regarding a method according to theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates, in simplified form, a system for detecting lineardimensions of workpieces 1 on a machine tool, for example a machiningcentre, schematically shown and identified by reference number 2 in thefigure, where the pieces are machined. The system comprises, apart froma processing, display and control unit, in particular a computerizednumerical control unit 13, that supervises the functioning of themachine tool 2, a detecting apparatus with a detecting head 3. Thelatter has a support portion 4, coupled to the slides of the machinetool 2, a feeler 6 and an arm 5 carrying the feeler 6 and moving withrespect to the support portion 4. The specific arrangement ensures thatthe support portion 4 of head 3 and the workpiece I be mutually movable.Head 3 also comprises detecting and transmitting devices with at least agenerator of electromagnetic signals 8, in particular a light source 7for emitting infrared optical radiations suitably coded, or modulated,for example, further to contact occurring between the feeler 6 and theworkpiece 1, and containing other information like the extent to whichan electric power supply battery (not shown in the drawing), housed inthe head, is charged. Conversion devices, in particular optoelectronicconverters comprising a photodiode 9, are also located in head 3, andtransform electromagnetic radiations, or optical signals, 10 ("switchon" signals arriving from a transceiver unit 11 that will be describedhereinafter) into electric signals suitably utilized for changing thepower supply condition of head 3 in a per se known way; substantiallyfor connecting the transmitting devices housed in the head to thebattery.

The detecting apparatus also comprises the previously mentionedtransceiver unit 11, and an interface unit 12. The transceiver unit 11is placed at a distance from head 3, and substantially houses areceiving section for receiving the radiations 8, and a transmittingsection for transmitting the radiations 10.

Interface unit 12 is connected to transceiver unit 11, and is in turnconnected to the numerical control unit 13 that also receivessignals--in a known way, not shown in the figure--indicative of themutual position existing between head 3 and workpiece 1, for providinginformation regarding that position at the moment when contact occurs.

Transceiver unit 11, schematically shown in FIGS. 2 and 3, comprises asubstantially cylindrical casing 14, and a cover 15 with a circularopening, fixed to casing 14 by means of a threaded coupling. Aprotection element, more specifically a glass disk 16, closes theopening and is fixed by way of a first threaded ring nut 17, coupled tothe internal wall of cover 15.

A metal cup 18 is housed in casing 14 and a support 19 for electricprinted circuits is mounted within the interior of cup 18. A metal plate20 with a substantially circular shape is fixed to casing 14, by way ofa second threaded ring nut 21, to close the opening of cup 18, thusfeaturing an electromagnetic screening for the circuits of support 19.

Support 19 carries, among other components, devices for theoptoelectronic conversion with photodiodes connected in parallel andlight emitting diodes ("LEDs"). More particularly, in the example shownin FIGS. 2 and 3 there are foreseen seven photodiodes 22 and seven LEDs23-29. The photodiodes 22 are alike and placed one beside the other, andtransform the optical radiations that they receive into electric signalsthat are thereon suitably amplified and processed by circuits of support19 and of interface unit 12. LEDs 23-29 accomplish the followingfunctions: visually monitor that the transceiver unit 11 is on (LED 23);visually monitor that a signal has been received by the same unit 11(LED 24); send switch on signals 10 or other signals (LEDs 25-28); andprovide a visual indication that unit 11 is sending signals (LED 29). Anelement 30 made of metal sheet (for example, of copper), consisting oftwo parts, provides a further electromagnetic screening of thephotodiodes 22 and of other circuits of support 19 at both sides of thesame support 19, and defines seven openings at positions correspondingto the photodiodes 22, for enabling the passage of the light signals.

The metal plate 20 defines holes for the passage of LEDs 23-29 and acircular opening located at a position corresponding to the photodiodes22, that is closed by a film 31 providing a light filter for allowingthe passage of only those radiations that are comprised within theinfrared spectrum and preventing other light radiations, also present inthe environment and that could generate malfunctions in unit 11, fromentering.

Unit 11 does not have focusing elements, nor optical amplificationelements, like lenses or concave mirrors, and the infrared radiations 8,transmitted by the light source 7 of head 3, are received, through theglass disk 16, directly by the seven photodiodes 22 that provide,together, a broad sensitive surface.

FIG. 4 shows, in an extremely simplified form, the main circuits locatedon support 19 of the transceiver unit 11, and in the interface unit 12.Each of these circuits may be implemented and function in per se knownways that are not detailedly disclosed in the present description.

The transceiver unit 11 includes an amplifying circuit 33, connected tophotodiodes 22, a clipping circuit 34, connected to amplifying circuit33, and a driving circuit 35, connected to LEDs 25-28, for enabling unit11 to perform transmission of signals.

Interface unit 12 comprises means for generating electric signals--inparticular circuits for generating switch on signals 37 and circuits forgenerating test signals 38--and reception means comprising decodingcircuits 39. Moreover, interface unit 12 comprises switching means, witha switch 40, and LEDs 41, 42 for visually monitoring some of theinformation contained in the signal received by unit 11 (LED 41), likethe contact occurring between feeler 6 and workpiece 1 and the chargingextent of the battery arranged within the head, and for monitoring anypossible error in the transmission (LED 42), whenever "valid" signalsare not being received for a certain time interval (for example 40 ms),in other terms coded signals containing information transmitted by head3. The decoding circuits 39 are connected, from one side to the clippingcircuit 34 of unit 11 through connection means with a first connectionline 43 and on the other side to the processing unit 13, in addition toLEDs 41 and 42. Numerical control unit 13 receives information containedin signal 8 further to reception by the photodiodes 22, suitableamplification (33) and clipping (34), as well as information obtainedthrough decoding operations performed by circuits 39. Circuits 37 areconnected to processing unit 13 and, by means of selector switch 40 andconnection means including a second connection line 44, to the drivingcircuit 35 of the transceiver unit 11. A control, arriving from thenumerical control unit 13, gives rise to a switch on signal generated bycircuits 37 and sent to the driving circuit 35 for generating theassociated optical signals 10 by LEDs 25-28.

Circuits 38 are also connected--alternatively to circuits 37--to thedriving circuit 35 of the transceiver unit 11 by means of selectorswitch 40 and the second connection line 44.

The two connection lines 43 and 44 include electric conductorscomprising wires of a cable 45 connected to units 11 and 12 by means ofterminal pins schematically shown for simplification purposes andidentified by reference numbers 46, 47, 48 and 49. Terminal pins 46 and47 provide the coupling between the wires of cable 45 and unit 11 in sofar as the first (43) and the second (44) connection lines areconcerned, respectively, whereas terminal pins 48 and 49 provide asimilar coupling between the wires of cable 45 and interface unit 12.

Under normal operating conditions, selector switch 40 is in a position A(shown by a dashed line in FIG. 4), for connecting the circuits 37 tothe second connection line 44. When numerical control unit 13 providesan appropriate control, a signal for switching on head 3 is generated ininterface unit 12 and is optically sent through the LEDs 25-28 (signal10, shown by a dashed line in FIG. 4), as hereinbefore alreadymentioned. Further to the photodiode 9 receiving the radiations 10, thetransmission devices of head 3 are power supplied (in a known way, thatdoes not directly concern this invention), and can transmit suitablymodulated, or coded, optical signals 8 (indicated by a dashed line inFIG. 4) indicative, for example, of the position of feeler 6, or of theremaining level of charge of the battery arranged on the head.

When the system fails to operate correctly in the course of the usualoperations and LED 42 monitors an error in the transmission, it ispossible to rapidly identify which element is causing the fault byfollowing the procedure hereinafter described by making reference to theblock diagram shown in FIG. 5.

Block 50: the system is set up to perform the testing procedure byscreening the transmission device of head 3, in order to prevent signals8 from reaching, in this phase, the transceiver unit 11.

Block 51: selector switch 40, possibly of manual type, is displaced to aposition B, thus causing the sending of a test signal, generated bycircuits 38, through the conductors of the second line 44, to thedriving circuit 35: an associated electromagnetic test signal, morespecifically, an optical signal 8' is emitted by LEDs 25-28. The opticalsignal 8' differs from the switch on signal 10, and is under all aspectssimilar to a signal 8 sent by the transmitter of head 3. The test signalgenerated by circuits 38 is also sent to the clipping circuit 34, thatconsequently modifies the associated comparison thresholds in order todiminish the overall sensitivity of the receiving section of unit 11.

Block 52: the optical signal 8' is partially reflected by the surface ofthe glass disk 16 of transceiver unit 11, and received by the receivingsection of unit 11, i.e. by photodiodes 22. Further to simpleprocessings in the amplifying circuit 33 and clipping circuit 34 of thereceiving section, an associated signal is sent, through the conductorsof the first line 43, to the decoding circuits 39 of interface unit 12.

Block 53: the status of the monitoring LED 42 is checked: if it is on,i.e. it stands to indicate there is an error in the reception of signal8', this means that there is a fault in the assembly comprisinginterface unit 12, cable 45 and transceiver unit 11; the testing withinthis assembly continues (see block 55).

Block 54: if LED 42 is off, i.e. no error is monitored in thetransmission, then it is inferred that there is a fault in head 3, owingto the fact that it is the only part of the system excluded from thistesting phase.

Block 55: the transceiver unit 11 is insulated, by disconnecting theends of the wires of cable 45 from terminal pins 46 and 47, and byconnecting to each other these ends, in order to shortcircuit the first(43) with the second (44) connection lines: in this way the test signal,generated in the interface unit 12, is electrically sent to thereception means 39 of unit 12.

Block 56: the monitoring LED 42 is checked again: if it is still on,i.e. if there is an error in the reception of the test signal, thenthere is a fault in the assembly comprising interface unit 12 and cable45; the testing of this assembly continues (see block 58).

Block 57: if LED 42 is off, i. e. no error is monitored in thetransmission, it is inferred that there is a fault in the transceiverunit 11, for the reason that it is the only part of the faulty assemblythat is being excluded from the test in this phase.

Block 58: cable 45 is disconnected from the interface unit 12 too, andterminal pins 48 and 49 are connected to each other: in this case, too,the test signal, generated in the interface unit 12, is electricallysent to the reception means 39 of the unit 12.

Block 59: the monitoring LED 42 is checked again.

Block 60: if LED 42 is off, i.e. no error is monitored in thetransmission, it is inferred that the fault lies in the cable 45, sincethis is the only part of the faulty assembly that is being excluded fromthis testing phase.

Block 61: if LED 42 is on, i.e. an error is monitored in the receptionof the test signal, then the fault is to be found in interface unit 12,for the reason that it is the only part undergoing the test in thisphase.

Block 62 indicates the end of the test procedure.

The electromagnetic test signal 8' transmitted by LEDs 25-28, further toits generation in circuits 38, has features that are by all meanssimilar to those of an optical coded signal 8 transmitted by lightsource 7 of head 3. This is necessary because, in order to test thecorrect operation of the part of the system comprising transceiver unit11, interface unit 12 and cable 45, it is not sufficient to use theswitch on signal 10 generated by circuits 37. As a matter of fact, theswitch on signal 10 is generally a plain impulse and in any case doesnot contain coded information that can be identified by reception means39; consequently, if possibly the receiving section of the transceiverunit 11 receives a switch on signal 10, interface unit 12 does notidentify any "valid" reception signal and indicated an error (LED 42on).

It should be realized that in the illustrated example, even theconnection lines 43 and 44 have identical electrical features: thisenables to shortcircuit lines 43 and 44 by connecting to each other thewires of cable 45 and/or the terminal pins 48 and 49, according to thehereinbefore described phases identified by blocks 55 and 58 in FIG. 5.

As the receiving and the transmitting sections of the transceiver unit11 are, in practice, very close together, it is practically inevitablethat there occurs a capacitive coupling between the two sections(schematically shown by reference number 36 in FIG. 4) that can cause,in the course of the initial testing phase (blocks 51-53), the passageof the test signal to the receiving section through an unwanted path,thus running the risk of excluding the photodiodes 22 and LEDs 25-28from the test.

In order to overcome this inconvenience, the overall sensitivity of thereceiving section is diminished so that it is not substantially in acondition of detecting relatively weak signals as those that might reachit in an anomalous way (through the capacitive coupling), but cancorrectly receive just the definitely stronger signals arriving via LEDs25-28, more specifically electromagnetic test signals 8' reflected by asurface located next to said LEDs 25-28 (for example at a distance ofless than 20-30 cm from them). The attenuation means that provide, inpractice, the decrease of the sensitivity comprise a suitable connectionbetween the second transmission line 44 and the clipping circuit 34. Aspreviously described (block 51), if the test signal is sent to anappropriate input of the clipping circuit 34, it causes, in a per seknown way, a shifting of the comparison threshold of said circuit 34until the test signal is transmitted, and causes the desired decrease ofsensitivity in the course of this phase of the testing procedure.

It should be realized that, under the normal operating conditions of thesystem, the connection of the second connection line 44 to an input ofthe clipping circuit 34 causes the sensitivity of the receiving sectionto diminish when sending a switch on signal: however, it is evident thatthis does not affect in any way the correct functioning of the systemas, during the switch on phase, the features of the receiving section ofunit 11 are unimportant.

It should also be realized that there exist other known ways, differingfrom the one hereinbefore described, for diminishing the sensitivity ofthe receiving section in the course of the testing phase, for example,by means of circuits adapted for reducing, during said phase, the gainof amplifier 33.

The use of glass disk 16 for partially reflecting the optical testsignal 8', emitted by LEDs 25-28, towards the photodiodes 22 (block 52)is particularly convenient, but obviously is not a limitation of themethod according to the present invention. In the absence of said glassdisk, it is possible to apply a different obstacle--like a sheet ofpaper, or a hand--in order to provide a suitable reflecting surface.

For an even more accurate test of the operability of the system, therecan be performed further additional tests, for example, to test that, inthe absence of reflecting surfaces facing LEDs 25-28, and thus in theabsence of optical test signals 8', received through the photodiodes 22,an error be monitored by the interface unit 12 (LED 42 on). In thecontrary, there could be other reasons determining the malfunction,differing from those that can be tested by following the procedure shownin FIG. 5 (for example a sending back of the test signal owing to thecapacitive effect 36, or through other anomalous ways).

There can be foreseen, according to a known method and without makingany substantial changes to the system, the possibility of sendingwireless "switch off" signals, similar to the "switch on" ones, forswitching off the connection between the transmission devices on thehead and the associated battery.

The herein described method involves manually performed operations, butthere can be foreseen the possibility of automating at least some phasesof the test (for example the starting of the procedure, the switchingfrom the circuit 37 generating switch on signals to the circuits 38 forgenerating test signals, . . . ).

Moreover, the circuits, the connections and the whole configuration,shown in a very schematic way in FIG. 4, can be made in different ways.More particularly, and just for the purpose of providing an example: themeans 37, 38 for generating the switch on signals and the test signals,and the switching means 40 can be part of a single element of thecircuit; the connection lines 43 and 44 can comprise elements that aredifferent from the electric wires of cable 45, like fiber optics; theinterface unit 12 can be integrated in the transceiver unit 11, i.e.suitable elements of the latter can accomplish functions that havepreviously been herein described with reference to the circuits of unit12.

A different configuration of the system can foresee the use, as a testsignal 8', of the actual switch on signal 10. In this case, in interfaceunit 12 circuits 38 and switch 40 are not present, and the receptionmeans (39) are modified and can detect, further to a suitable switching,when the features of the signal received subsequently to the reflectionare substantially identical to those of the transmitted switch onsignal.

The test procedure can also be applied, without there being the need forany substantial change, to systems that use the wireless transmission ofsignals different from optical signals (for example radiofrequencysignals).

We claim:
 1. System for checking the dimensions of mechanical workpieces(1), including a detecting apparatus and a processing and control unit(13), the detecting apparatus comprising:a detecting head (3) forcooperating with the workpiece (1) to be checked, with transmissiondevices (7) for transmitting first electromagnetic coded signals (8), atransceiver unit (11), separate from the head, with a receiving section(22,33,34) for receiving the first electromagnetic coded signals (8),and a transmitting section (25-28,35) for transmitting secondelectromagnetic signals (8',10), and an interface unit (12), connectedto the transceiver unit (11) and to the processing and control unit(13), comprising reception means (39) and means for generating switch onsignals (37), characterized in that said interface unit (12) alsocomprises means for generating test signals (38), the transmittingsection (25-28,35) and the receiving section (22,33,34) of thetransceiver unit (11) being adapted for transmitting associatedelectromagnetic test signals (8') and for receiving reflectedelectromagnetic test signals (8'), respectively, for testing theoperability of the detecting apparatus.
 2. A system according to claim1, wherein the detecting apparatus comprises connection means (45)between the transceiver unit (11) and the interface unit (12), with afirst connection line (43) between said receiving section (22,33,34) andsaid reception means (39), and a second connection line (44), theinterface unit (12) comprising switching means (40) for connecting, bymeans of the second connection line (44), the means for generatingswitch on signals (37) and, alternatively, the means for generating testsignals (38) to said transmitting section (25-28,35).
 3. A systemaccording to claim 2, wherein said receiving section (22,33,34)comprises attenuation means connected to the second connection line (44)for decreasing the reception sensitivity of said receiving section(33,34).
 4. A system according to claim 2, wherein the detectingapparatus comprises a cable (45) with electric wires for electricallyconnecting the transceiver unit (11) and the interface unit (12), saidfirst (43) and second (44) connection lines comprising said wires of thecable (45).
 5. A system according to one of claims 1-4, wherein thereceiving section (22,33,34) and the transmitting section (25-28,35) ofsaid transceiver unit (11) comprise devices for the optoelectronicconversion (22,25-28), said electromagnetic signals (8,8',10) being ofoptical type.
 6. A method for testing the operability of a system forchecking the dimensions of mechanical workpieces comprising a detectinghead (3) for transmitting electromagnetic coded signals (8), andinterface and transceiver means (11,12), separate from the head, with areceiving section (22,33,34) for receiving electromagnetic signals (8),means (38) for generating test signals, and a transmitting section(25-28,35) for transmitting electromagnetic signals (8',10), the methodcomprising the steps ofcommanding (51) the interface and transceivermeans (11,12) to generate test signals and to transmit associatedelectromagnetic test signals (8'), arranging a reflecting surface (16)next to said transmitting section (25-28), so as to cause (52) thereflection of the electromagnetic test signals (8',10) towards thereceiving section (22) of the interface and transceiver means (11), andtesting (53,54) that the receiving section (22,33,34) has correctlyreceived the reflected electromagnetic test signals (8',10).
 7. Themethod according to claim 6, for testing the operability of a checkingsystem, comprising a processing and control unit (13), the interface andtransceiver means including a transceiver unit (11) and an interfaceunit (12), the latter being connected to the processing and control unit(13) and to the transceiver unit (11) and comprising reception means(39) and said means (38) for generating test signals, the methodcomprising the step of connecting (51) said means (38) for generatingtest signals to said transmitting section (25-28,35), for obtaining thetransmission of said electromagnetic test signals (8') by thetransceiver unit (11) and the reflection (51) of said electromagnetictest signals (8'), and for testing (53,54) that the reception means (39)have correctly received the reflected signals.
 8. The method accordingto claim 7, for testing the operability of a system wherein thetransceiver unit (11) comprises a casing (14) and a glass element (16)mechanically coupled to the casing (14), wherein the glass element (16)defines said reflecting surface.
 9. The method according to claim 7, fortesting the operability of an apparatus with connection means betweenthe transceiver unit (11) and the interface unit (12) including a first(43) and a second (44) connection line, the method comprising thefurther steps ofinterrupting (55,58) the connection between thetransceiver unit (11) and the interface unit (12), shortcircuiting(55,58) to each other said first connection line (43) and said secondconnection line (44), and testing (56,57,59,60,61) the correct receptionof the test signals, transmitted through the connection lines (43,44)that are shortcircuited, by the reception means (39) of the interfaceunit (12).
 10. The method according to one of the preceding claims 6 to9, characterized by the further step of causing a decreasing in thesensitivity of the receiving section (22,33,34) of the interface andtransceiver means (11).